1. Field of the Invention
The present invention relates to an apparatus for fault detection for parallelly transmitted audio signals and an apparatus for delay difference detection and adjustment for parallelly transmitted audio signals, and more particularly to an apparatus for fault detection for parallelly transmitted audio signals suitable for selective reception of fault-free audio signal at reception side by transmitting a same audio signal by dividing into two channels, and an apparatus for delay difference detection and adjustment for parallelly transmitted audio signals transmitted in two channels.
2. Description of the Related Art
Hitherto, in parallel transmission of television signals for the purpose of high reliability transmission, same video/audio signals are transmitted by dividing into two channels, and either channel is selected and received at the reception side.
FIG. 6 is a block diagram showing such a conventional parallel transmitting apparatus. The diagram shows only about the audio signal. The upper side in the diagram is supposed to be the working system, and the lower side is the standby system, and the reception side manual switch 3 is usually connected to select the working system. At this time, the input audio signal is transmitted by way of encoder 1-1, working transmission line 4-1, decoder 2-1, and manual switch 3, and becomes an output audio signal.
An inspector monitors the channel status of two systems at the reception side, and when detecting an occurrence of a fault in the working system, the inspector switches the manual switch 3 to the standby side. As a result, the input audio signal is transmitted by way of encoder 1-2, standby transmission line 4-2, decoder 2-2, and manual switch 3, and the audio signal is received intermittently through a normal channel.
The present applicant has previously proposed a technique of minimizing the fault time of output signal by automating switching to the normal channel in the event of a fault occurring in the receiving circuit during parallel transmission in the following patent reference 1. A technique for detecting and adjusting delay difference of video signals has been proposed in the following patent reference 2.
[Patent reference 1] Japanese Patent Laid-Open (JP-A) No. 2000-350238
[Patent reference 2] Japanese Patent Laid-Open (JP-A) No. 2002-142131
If the inspector manually switches the switch to select a normal channel to receive video/audio signals intermittently, it takes too much time until completion of switching to normal channel capable of receiving normal audio signals, and switching without interruption is difficult, and moreover the audio signal may not be continued smoothly at the time of switching due to delay and deviation of audio signals between two channels.
The technique in patent reference 1 relates to video signals, and it is intended to switch to a normal channel by detecting fault in video signals. It is based on the condition that images of two systems entering the apparatus have been already matched in position on the screen, being free from delay deviation between the images. However, if the images are normal, the audio only may be faulty. In the technique of patent reference 1, the switching toward the normal channel side is not performed in such a case, and the audio signal remains defective.
In parallel transmission, in most cases, the two systems run different routes geometrically, and generally there is a delay difference between images of two systems. The delay difference between images of two systems can be adjusted in the technique of patent reference 2, but this is intended to detect and adjust the delay difference of video signals. When each system comprises devices for compressing and decoding television signals, video and audio signals are processed separately, and the processing timing may be deviated slightly, and if the delay difference of video signal is adjusted, the delay difference of audio signal is not adjusted, and a slight deviation may be left over. Therefore, in order to receive audio signals continuously and favorably regardless of switching of systems, it is also required to adjust the delay difference between audio signals in two systems.